1. Field of the Invention
The present invention generally relates to manufacturing semiconductor circuits via photolithography and, more particularly, but not by way of limitation, to a method for photomask making that applies a defect finder mark to a photomask in the vicinity of a defect during inspection so that the defect can be reliably identified and eliminated from the photomask during repair.
2. Description of the Related Art
Photolithography is a well-known process in the semiconductor manufacturing industry for transferring photographic images of complex circuit patterns onto the surface of a semiconductor wafer substrate. Typically, photolithography requires passing light through a photomask containing circuit patterns to cast an image of that photomask onto the semiconductor wafer substrate.
In particular, the photomask includes a mask substrate and a mask pattern disposed on the mask substrate. Commonly, the mask substrate is composed of a transparent material, such as glass for example, whereas the mask pattern is composed of an opaque material, for example chrome or an emulsion film having opaque particles set therein, such as silver particles. During photolithography, light passes from a light source through the photomask via the mask substrate so that the surface of the semiconductor wafer substrate is exposed. This surface features a uniform layer of photoresist thereon that develops in response to irradiation. The throughput of light across the photomask forms an image on the photoresist that corresponds to the mask pattern. Accordingly, the resulting image is etched so that the semiconductor wafer substrate ultimately forms at least one integrated circuit and/or microcircuit.
Commonly in the industry, each photomask is manufactured by a front-end method and a back-end method for photomask making. Generally, in a manner similar to forming images on the surface of a semiconductor wafer substrate, the front-end method for photomask making forms the desired mask pattern onto the mask substrate. In particular, images for forming the mask pattern are generated by a layout program. An irradiating device maps the images from the layout program onto the mask substrate so that a layer of photoresist on the surface of the mask substrate is exposed to the radiation from the irradiating device.
In particular, the surface of the mask substrate includes a uniform layer of chrome disposed on the mask substrate and a uniform layer of photoresist disposed on the layer of chrome. Responsive to the layout program, the irradiating device forms an image on the photoresist that corresponds to the mask pattern. Typically, the developed image is etched so that the chrome layer thus defines the mask pattern and so that the mask substrate is exposed to facilitate throughput. Subsequent to etching, the front-end method for photomask making further requires removal of photoresist from the photomask.
Generally, to eliminate defects on the photomask arising from the front-end method, the back-end method for photomask making implements quality control procedures. In effect, mitigating or eliminating defects from the photomask with the back-end method reduces the number of defects on semiconductor wafer substrate created from the photomask with the photolithography process.
Commercially available photomask inspection apparatuses are typically used to inspect the photomask for defects. Each photomask is placed on a platform or, commonly, “stage”. This stage enables a technician to move the photomask with precision when inspecting the photomask for defects. Photomask inspection apparatuses typically include field of view displays for enabling technicians to visually identify defects. The resolution of the field of view display varies with each photomask inspection apparatus.
Once defects are identified, the photomask is removed from the photomask inspection apparatus and, typically, placed atop a stage within a commercially available mask repair device. Often the task of relocating each defect on the mask repair device is inhibited due to differences in precision stage movement as well as resolution between the mask repair device and the photomask inspection apparatus.
Moreover, defect relocation may be inhibited due to the kind of defect that was initially located. In practice, technicians often readily identify isolated or relatively large defects associated with each photomask. It is, however, very difficult for a technician to spot a small defect or a defect residing in a dense and repetitive pattern region of the mask pattern. Unfortunately, because of the difficulty and large quantity of time invested in locating small defects and/or defects within a dense mask pattern, today's technicians often overlook relocating and, thus, repairing such defects. Cumulatively, failing to repair such defects detrimentally effects the “electrical wafer yield” of the semiconductor wafer substrate that may, ultimately, result in operational failure of the resulting integrated circuit and/or microcircuit.
Accordingly, as a matter of reducing manufacturing time, labor, and cost, there is a long felt need for a method of photomask making that applies a defect finder mark to a photomask in the vicinity of a defect during inspection so that the defect can be reliably identified and eliminated from the photomask during repair.